1. Field of the Invention
The present invention relates generally to a transmission diversity detection circuit, a detection method thereof and a storage medium storing a transmission diversity detection program. More particularly, the invention relates to a transmission diversity detection circuit, a detection method thereof and a storage medium storing a transmission diversity detection program, applicable for spread spectrum communication performing transmission diversity, and especially applicable for the case where presence or absence of transmission diversity is notified by modulation of a SCH (Synchronization channel).
2. Description of the Related Art
A transmission diversity to perform transmission from a plurality (for example, two) of transmission antennas provided in one base station, and to receive transmitted data by a mobile station, such as a portable terminal or the like, is known. In such transmission diversity, since a plurality of paths between the base station and a mobile station are established, communication can be performed even when receiving condition in one path is not good, if receiving condition of another path is good.
A transmission diversity system applicable for spread spectrum communication has been disclosed in 3GPP (3rd Generation Partnership Project) specification TS25.211 V3.0.0 (TSGR1#7(99)g0). This transmission diversity system will be discussed with reference to FIGS. 7 to 13.
In FIG. 7, there is shown a transmission pattern of a primary CPICH (common pilot channel) symbol in the transmission diversity. In the shown example, it is assumed that transmission is performed using two transmission antennas 1 and 2. As shown in FIG. 7, from the antenna 1, “A” is transmitted continuously. On the other hand, from the antenna 2, “A”, “A” and “−A”, “−A” are transmitted alternately. Here, A is a symbol 1+j.
In the shown example, one frame (frame) is consisted of fifteen time slots (hereinafter, merely called as “slot”) #0 to #14. Accordingly, since one frame consists of an odd number of slots, “−A” and “A” are transmitted from the antenna 2 at the boundary of the frame PB (frame boundary). At portions other than the boundary FB, “A”, “A” and “−A”, “−A” are transmitted alternately from the antenna 2 as set forth above.
It should be noted that, when transmission diversity is not performed, the primary CPICH symbol is not transmitted on the side of the antenna 2, and only primary CPICH symbol on the side of the antenna 1 is transmitted.
In FIG. 8, there is shown a transmission pattern of the SCH. The SCH represents both of a primary SCH and a secondary SCH. Respective SCHs are those, in which the symbol 1+j is spread with a primary synchronization code and a secondary synchronization code. These SCHs are further modulated with “a”.
In FIG. 8, there is shown the slots #0 to #14 forming one frame. It is assumed that a period Tslot of the slot is 2560 chips. On the other hand, a period Tframe is 15×Tslot. Then, in one slot, after transmission of 256 chips of the primary SCH and the secondary SCH, data portion indicated by a primary CCPCH (Common Control Physical Channel) is transmitted.
“Cp” in the primary SCH is a primary synchronization code. On the other hand, “Cs” in the secondary SCH is a secondary synchronization code. It should be noted that “Csi,k” (k=0 to 14) represents that a code group number, in which a scramble code used in the base station, is “i”.
Here, “a” takes a value of “1” or “−1” according to the following condition. Namely, concerning the data portion indicated by primary CCPCH in FIG. 8, when transmission diversity is performed in a method called space time block coding based transmit antenna diversity (STTD), “a” is “1” and when STTD transmission diversity is not performed, “a” becomes “−1”.
Hereinafter, discussion will be given for STTD transmission diversity. FIG. 9 is a block diagram showing a construction of a primary portion of the base station for performing STTD transmission diversity. The base station is constructed with a STTD encoder 41 for inputting a quadrature phase shift keying (QPSK) symbol, a multiplexer (MUX) 42 being input an encoded output of the encoder 41, a pilot signal and a diversity pilot signal, a multipliers 43a and 43b for spreading an output of the multiplexer 42 with a scramble code C, and antennas 1 and 2 provided corresponding to the multipliers 43a and 43b. By utilizing the base station having such construction, the STTD transmission diversity described in the specification set both above is performed.
Transmitting operation of the base station with the construction set forth above will be discussed with reference to FIG. 10. The STTD encoder 41 converts input symbol as shown in FIG. 10. In FIG. 10, among input signal to the STTD encoder 41, in a front portion of the data portion Ndata, symbols S1 and S2 are present. Namely, during a period from a time 0 to a time T, symbol S1 is present, and during a period from the time T to a time 2T, the symbol S2 is present.
Concerning these symbols S1 and S2, the STTD encoder 41 outputs symbols S1 and S2 as they are, to the side of the antenna 1 (Ant1). On the other hand, to the side of the antenna 2 (Ant2), instead of outputting symbols S1 and S2 as they are, a complex conjugate −S2* of the symbol S2 and a complex conjugate S1* of the symbol S1 are output alternately. As a result, during the period from the time 0 to time T, the symbol S1 is transmitted from the antenna 1. At the same time, the complex conjugate −S2* of the symbol S2 is transmitted from the antenna 2. On the other hand, during the period from the time T to the time 2T, the symbol S2 is transmitted from the antenna 1, and at the same time, the complex conjugate S1* of the symbol S1 is transmitted from the antenna 2.
Expressing the transmitting condition in an orthogonal coordinates, the symbol S1 and the complex conjugate −S2* are transmitted during the period from the time 0 to the time T as shown in FIG. 11A. On the other hand, the symbol S2 and the complex conjugate S1* are transmitted during the period from the time T to the time 2T as shown in FIG. 11B.
The following is a reason why the complex conjugate of the symbol is output from the antenna 2. For example, when the signal arriving from the antenna 1 during the period from the time 0 to the time T and the signal arriving from the antenna 2 during the period from the time 0 to the time T are situated in a relationship to weaken with each other, the signal arriving from the antenna 1 during the period from the time T to the time 2T and the signal arriving from the antenna 2 during the period from the time T to the time 2T are inherently situated in a relationship to strengthen with each other. Namely, as shown in FIG. 10, between two antennas of the base station and an antenna 3 on reception side, there are paths P1 to Pj (j is a natural number) are present as propagation paths. Signals received by the antenna 3 becomes a sum of symbols transmitted from the antennas 1 and 2 at the same timing and varied amplitudes and phases through a plurality of paths. Even if the signals received by the antenna is weaken due to variation of the amplitude and the phase caused by the paths during a certain period, i.e. either the period from 0 to T or the period from T to 2T, the arriving signals from a plurality of paths are strengthened with each other in another period, to increase probability of correct reception.
FIG. 12 shows a typical construction of the major part on a reception side in relation to the base station. Referring to FIG. 12, the reception side is constructed with a reception antenna 3, a demodulator (Q-DEM) 71 corresponding to the foregoing encoder and an A/D converter 27 which converts an analog signal Ia and Qa into digital signals Id and Qd. In a receiver of the construction set forth above, if transmission diversity is performed on the base station side, the reception can be correctly performed.
Here, FIG. 13 shows a transmission diversity pattern of the SCH. The transmission diversity system is those called as TSTD (time switched transmit diversity for SCH) and is different from the foregoing STTD transmission diversity. When the TSTD transmission diversity is not performed, the SCH is performed only from the antenna 1. Accordingly, irrespective presence or absence of the TSTD transmission diversity, the SCH is never transmitted from a plurality of antennas, simultaneously.
On the other hand, in the above-mentioned STTD transmission diversity, encoding is performed by means of the STTD encoder 41 shown in FIG. 10. Therefore, decoding by the decoder becomes necessary.
However, in either for STTD transmission diversity or TSTD transmission diversity, presence and absence of transmission diversity will not be preliminarily notified from the base station as transmission side. Namely, if there is a limitation in installation of the antennas or the like, transmission diversity is not always performed in all base stations, the presence or absence of the STTD transmission diversity has to be detected by the terminal as reception side. Therefore, whether transmission diversity is performed or not should be detected on the reception side.
Upon detection, in general, it may be merely required to predict direction of modulation “a” of the SCH. However, phase of the symbol through the propagation paths becomes indeterminative. Therefore, on the basis of phase difference with a known symbol to be a reference of the phase, the direction of modulation “a” of the SCH is predicted.
As a known symbol to be a reference of the phase, use of a primary CPICH symbol is considered. Namely, as shown in FIG. 14, by detecting a relative phase relationship between the primary CPICH symbol P and the SCH symbol S, the direction of modulation “a” of the SCHs can be predicted. Referring to FIG. 14, if the phase difference between the primary CPICH symbol P and the SCH symbol S is 0°, the modulation a=1, and if the phase difference is 180°, the modulation a=−1. However, since pilot pattern is different depending upon presence or absence of the transmission diversity, prediction cannot be done in straightforward.
In the structures of the slot and symbol as set forth above, in order to predict the phase of the SCH, it may be required to predict the propagation path on the side of the first antenna. In this case, in order to predict the propagation path on the side of the first antenna, at least a pair of pilot symbols having mutually opposite pilot patterns on the side of the second antenna are required.
However, when two symbols are used, if an error is present in a reference oscillation frequency between the base station and the mobile terminal, phase rotation is caused between the symbols. This makes it necessary to simultaneously perform prediction and correction of phase rotation, which makes the process very complicated.